THE DAWN OF LIFE IO7 



therefore, we find great and conformable beds of limestone, 

 such as those described by Sir William Logan in the Lauren- 

 tian of Canada, we naturally imagine a quiet sea bottom, in 

 which multitudes of animals of humble organization were 

 accumulating limestone in their hard parts, and depositing 

 this in gradually increasing thickness from age to age. Any 

 attempts to account otherwise for these thick and greatly 

 extended beds, regularly interstratified with other deposits, 

 have so far been failures, and have arisen either from a want 

 of comprehension of the nature and magnitude of the appear- 

 ances to be explained, or from the error of mistaking the true 

 bedded limestones for veins of calcareous spar. 



The Laurentian rocks contain great quantities of carbon, in 

 the form of graphite or plumbago. This does not occur 

 wholly, or even principally, in veins or fissures, but in the sub- 

 stance of the Hmestone and gneiss, and in regular layers. So 

 abundant is it, that I have estimated the amount of carbon in 

 one division of the Lower Laurentian of the Ottawa district at 

 an aggregate thickness of not less than twenty to thirty feet, an 

 amount comparable with that in the true coal formation itself. 

 Now we know of no agency existing in present or in past 

 geological time capable of deoxidizing carbonic acid, and 

 fixing its carbon as an ingredient in permanent rocks, except 

 vegetable life. Unless, therefore, we suppose that there existed 

 in the Laurentian age a vast abundance of vegetation, either in 

 the sea or on the land, we have no means of explaining the 

 Laurentian graphite. 



The Laurentian formation contains great beds of oxide of 

 iron, sometimes seventy feet in thickness. Here, again, we 

 have an evidence of organic action ; for it is the deoxidizing 

 power of vegetable matter which has in all the later formations 

 been the efficient cause in producing bedded deposits of iron. 

 This is the case in modern bog and lake ores, in the clay iron- 

 stones of the coal measures, and apparently, also, in the great 



